In many applications of electrical and electronic control systems a fail-safe capability is not only desirable but essential. A common characteristic of these applications is that human lives are dependent upon the control system. One class of these applications which comes readily to mind are control systems useful in a railroad environment, such as that of controlling the application of motor and braking power to automatic railroad vehicles in response to signals communicated from the wayside, controlling the clearing of wayside signals and the like. In this class of applications there is a well developed body of art based upon relay technology which enjoys the necessary fail-safe characteristics.
With the advent of solid-state components and circuitry the desirability of replacing the relay devices with solid-state circuits and components was obvious to many skilled in the art. The solid-state components has many qualities which were generally desirable in control system applications. The absence of moving parts meant that many types of relay failures would not be exhibited by the solid-state components. In addition, the absence of mechanically moving parts enabled the solid-state circuits to operate at much higher speeds than did the relay circuits. And finally, the lifetime of solid-state circuits was projected to be greater than that of the relay. However, the solid-state circuits and components also had undesirable qualities which, for many years have prevented their use as a replacement for the relays, especially in circuits where fail-safe capabilities were essential. The solid-state components were subject to catastrophic failures, some of which would necessarily result in a false output indication. Furthermore, false output indications could be generated by reason of leakage currents which, because of the nature of the solid-state components, could not be eliminated. For this reason, although solid-state components have replaced relays in many applications, there are few, if any, solid-state circuits in use today in which fail-safe capability is essential.
Notwithstanding the foregoing a number of solid state fail-safe logic gates have been proposed in the art. Typical of these is Thorne-Booth, U.S. Pat. No. 3,600,604. Thorne-Booth is typical of a number of logic gates which employ transformers to isolate the input and output of the gate. The difficulty with using transformers as an element which will be duplicated as many times as a logic gate is related to the cost and size of a transformer. At the frequencies of the dynamic signals employed herein the transformer in, for instance Thorne-Booth, will be by far the largest element in size. Absent for the necessity for such a transformer, the entire logic gate could, using conventional integrated circuit technology, be placed on a single chip. For at least this reason the cost and size of the Thorne-Booth transformer makes this solution undesirable.